Mixing C and C++ code yields unexpected behavior in GCC

Question

I use legacy C-Code in my current C++ project by including external headers:

extern "C" {
# include "ANN/ANN_4t70P1.h"
# include "ANN/ANN_4t70P2.h"
# include "ANN/ANN_4t70P3.h"
# include "ANN/ANN_4t70P4.h"
}

The header files look like this:

extern int ANN_4t70P1(float *in, float *out, int init);

static struct {
  int NoOfInput;    /* Number of Input Units  */
  int NoOfOutput;   /* Number of Output Units */
  int(* propFunc)(float *, float*, int);
} ANN_4t70P1REC = {8,3,ANN_4t70P1};

The C-Code is created by an ancient batch-file and cannot be compiled using C++ compilers. Nevertheless, this implementation works fine for Windows and Mac OS. However, when I compile the code using gcc and g++ on Linux and run the application, ANN_4t70P1REC returns incorrect values.

Are there any special linker flags that I missed out when linking the project?

Thanks!

ANN_4t70P1REC is an instance of a struct. What do you mean specifically when you say it "returns incorrect values"? — Vaughn Cato
– Vaughn Cato, Commented Aug 26, 2012 at 16:24
Maybe you need it all to be compiled in 32 bit? Newer machines could be installed as 64 bit so you might need to add the -m32 compiler flag — epatel
– epatel, Commented Aug 26, 2012 at 17:19
Sorry. Of course I mean the function propFunc within the instance ANN_4t70P1REC that returns unexpected results. The values do not seem to be random values or unallocated memory, but they are completely different from the values that I get with XCode or VC++. — bboverflow
– bboverflow, Commented Aug 27, 2012 at 9:26
Make a small C-only test program, and check if that returns correct values. — jpa
– jpa, Commented Aug 27, 2012 at 10:18
You don't have something odd like storing *propFunc in an int somewhere? On 64-bit machines, the pointer will be 8 bytes, not 4. As suggested, try -m32 and see if that makes a difference. — casualcoder
– casualcoder, Commented Aug 28, 2012 at 0:25

nmenezes · Accepted Answer · 2012-09-18 11:30:10Z

1

What do you mean by:

The C-Code is created by an ancient batch-file and cannot be compiled using C++ compilers

Are you linking using object files generated by different compilers? If so, try to inspect your object files with:

readelf -h <objectname>

Check if there is a different ABI. If the code is generated by a very old GCC <3.3/3.4 you can have problems linking with newer versions.

Are you sure you don't have any warnings during the link?

answered Sep 18, 2012 at 11:30

nmenezes

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